1. Field of the Invention
This invention relates generally to read/write head arrays for magnetic data stores and more particularly to a monolithic magnetic read-while-write tape head.
2. Description of the Prior Art
Business, science and entertainment applications depend upon computers to process and record data, often with large volumes of the data being stored or transferred to nonvolatile storage media, such as magnetic discs, magnetic tape cartridges, optical disk cartridges, floppy diskettes, or floptical diskettes. Typically, magnetic tape is the most economical means of storing or archiving the data. Storage technology is continually pushed to increase storage capacity and storage reliability. Improvement in data storage densities in magnetic storage media, for example, has resulted from improved medium materials, improved error correction techniques and decreased areal bit sizes. The data capacity of half-inch magnetic tape, for example, is now measured in gigabytes on 384 or more data tracks.
The improvement in magnetic medium data storage capacity arises in large part from improvements in the magnetic head assembly used for reading and writing data on the magnetic storage medium. A major improvement in transducer technology arrived with the magnetoresistive (MR) sensor originally developed by the IBM Corporation. The MR sensor transduces magnetic field changes in a MR stripe to resistance changes, which are processed to provide digital signals. Data storage density can be increased because a MR sensor offers signal levels much higher than those available from conventional inductive read heads for a given bit area. Moreover, the MR sensor output signal depends only on the instantaneous magnetic field intensity in the storage medium and is independent of relative sensor/medium velocity and the magnetic field time-rate-of-change.
The quantity of data stored on a magnetic tape may be increased by increasing the number of data tracks on the tape, which also decreases the distance between adjacent tracks. The width of the data tracks written by the read/write head assembly is limited by the width of the magnetic pole pieces in the write element but this track-width can be much narrower than the write element itself, which includes a write coil to energize the head gap. Present multi-channel tape recording systems achieve data track densities on the tape medium of twelve or more times the recording element density in the read/write head assembly.
In modem magnetic tape recorders adapted for computer data storage, the magnetic head assembly offers a read-while-write capability that is an essential feature for providing virtually error free magnetically stored data. Providing this bi-directional read-while-write capability usually requires fabrication and assembly of two or three separate modules. In the three-module approach, modules containing arrays of read heads are aligned with and assembled on both sides of a center module containing an array of write heads. Thus, for both directions of tape travel, the tape first passes over a center write-module to be written and then immediately passes over a closely-spaced read module for reading of the data just written. This approach requires independent finishing of the air-bearing surface (ABS) on each of the three modules and the precise assembly of the three modules into a single recording head. In the two module interleaved approach, two identical modules are fabricated, each containing an array of alternating read and write elements, starting with, say, a read element and ending with a write element. These modules are then assembled xe2x80x98face-to-facexe2x80x99 so that a read element always faces a write element. When the tape is moving one direction, half of the write elements are followed closely by a read element. When moving the other direction, the other half of the write elements are followed by read elements, thereby supporting bi-directional read-while-write operation providing immediate read back verification of the data written onto the tape medium. Alternatively, each module may contain xe2x80x9cpiggy-backxe2x80x9d elements consisting of combined read and write elements such as used in direct access storage device (DASD) heads to increase data rates.
By continually reading xe2x80x9cjust recordedxe2x80x9d data, the quality of the recorded data is verified while the original data is still available in the temporary storage of the recording system for reuse if needed. The recovered data is compared to the original data to afford opportunity for action, such as re-recording, to correct errors. Alternate columns (track-pairs) are thereby disposed to read-after-write in alternate directions of tape medium motion or in the other approach, all columns are written or read in parallel. Tape heads suitable for reading and writing on high-density tapes require precise alignment of the track-pair elements in the head assembly, as well as tight control of the skew of the head in the drive relative to the direction of tape travel. This latter requirement is eased by reducing the spacing between the read and write elements in each track-pair. Thus, higher-density data storage on tape requires tighter control of read and write element spacing tolerances along both the transverse and the track-line dimensions of the tape head.
FIG. 1 shows the air bearing surface (ABS) of a prior art embodiment of an interleaved magnetoresistive (MR) head assembly 10, where the read elements are marked xe2x80x9cRxe2x80x9d and the write elements are marked xe2x80x9cWxe2x80x9d. The write elements, exemplified by the write head 12 and the read elements, exemplified by the read head 14, are disposed in alternating fashion to form a single row of read/write track-pairs, exemplified by the R/W track-pair 12-14. As used herein, the term xe2x80x9calternatingxe2x80x9d is intended to include other formats. For example, one format provides that the odd-numbered heads H1, H3, H5 etc. are operative during forward tape movement, while the even-numbered heads H2, H4, H6 etc. are operative during the opposite direction of tape movement. Generally, the length of the magnetic tape medium 16 moves in either a forward or reverse direction as indicated by the arrows 18 and 20. Head assembly 10 is shown in FIG. 1 as if magnetic tape medium 16 were transparent, although such tape medium normally is not transparent. Arrow 18 designates a forward movement of tape medium 16 and arrow 20 designates a reverse direction. Magnetic tape medium 16 and interleaved MR head assembly 10 operate in a transducing relationship in the manner well-known in the art.
Each of the head elements in head assembly 10 is intended to operate over a plurality of data tracks in magnetic tape medium 16, as may be appreciated with reference to the data tracks T1, T9, T17, etc. in FIG. 1, which shows an exemplary 288-track scheme having a data track density on magnetic tape medium 16 of eight times the recording element density of R/W track-pairs H1, H2, . . . H36 in MR head assembly 10. Tracks T9, T25, . . . T281 may be written with one pass of magnetic tape medium 16 in direction 18 over even-numbered R/W track-pairs H2, H4, . . . H36 and then tracks T1, T17, . . . T273 written on a return pass of magnetic tape medium 16 over the odd-numbered R/W track-pairs H1, H3, . . . H35 by moving the lateral position of MR head assembly 10 in the direction of the arrow 21 by a distance equivalent to one track pitch (T1-T2), which is about 12% of the R/W track-pair spacing (H1-H2).
Interleaved MR head assembly 10 includes two thin-film modules 22 and 24 of generally identical construction. Modules 22 and 24 are joined together with an adhesive layer 25 to form a single physical unit, wherein the R/W track-pairs are aligned as precisely as possible in the direction of tape medium movement. Each module 22, 24 includes one head-gap line 26, 28, respectively, where the individual R/W gaps in each module 26, 28 are precisely located. Each thin-film module 22, 24 includes a separate substrate 30,32 and a separate closure piece 34, 36, respectively. Substrate 30 is bonded near head-gap line 26 by adhesive to closure piece 34 to form thin-film module 22 and substrate 32 is bonded near head-gap line 28 by adhesive to closure piece 36 to form thin-film module 24. As precisely as possible, head-gap lines 26, 28 are disposed perpendicular to the directions of tape medium movement as represented by arrows 18, 20. The R/W head-gaps at H1-H36 in thin-film module 22 cooperate with the corresponding R/W head-gaps in thin-film module 24 to provide read-after-write functionality during movement of magnetic tape medium 16. The read head gaps of one thin-film module are precisely aligned with the write head gaps of the other module along the direction of movement of tape medium 16. Thus, for example, write head 12 is aligned with read head 14 to form a single R/W track-pair for read-after-write during magnetic tape movement in the direction indicated by arrow 18.
Interleaved MR head assembly 10, as known in the art, is disadvantageously limited to a relatively large spacing between the cooperating read and write heads in each R/W track-pair because of the two closure pieces 34 and 36 required to cover each of the two substrates 30 and 32 as discussed above with respect to FIG. 1. Also, the two thin-film modules 22 and 24 (FIG. 1) must be aligned along the track line of motion of the tape medium and joined together within a small fraction of the data track width, which is a slow and expensive process with high spoilage rates during manufacture.
An alternate embodiment known in the art to the interleaved MR head structure described above with reference to FIG. 1 is the xe2x80x98piggy-backxe2x80x99 structure wherein a write element is fabricated using thin-film techniques immediately over a MR read element on each side of every R/W track-pair H1, H2, etc., so that the R and W elements shown in FIG. 1 all become xe2x80x9cW piggy-backed-on Rxe2x80x9d elements. Within each piggy-back pair, the MR read head is too close to the overlying write head to permit reading while writing, but the MR read head on the other side of the R/W track-pair is aligned with the writing head and is distant enough to read while writing. The disadvantages of the interleaved MR head discussed above apply also to the xe2x80x9cpiggy-backxe2x80x9d head known in the art. In an alternative prior art embodiment (not shown), the two modules 22, 24 are bonded at the substrates with the closure layers disposed outermost. Such a structure has similar disadvantages.
The present state of the art requires independent finishing of the air bearing surfaces for two or three modules to submicron precision and the independent precise assembly of these modules into a recording head, with much cost and difficulty. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.
This invention solves the above problem by providing a method for fabricating multi-track thin-film magnetoresistive (MR) tape heads with precisely-aligned read/write (R/W) track-pairs fabricated on a monolithic substrate. As used herein, the term monolithic denominates an undivided seamless piece. The tape heads of this invention are fabricated from a single substrate wafer by using complete thin-film processing on both sides of the wafer. In a preferred embodiment, the head elements are aligned with readers opposite writers on the opposing sides of the monolithic substrate wafer. Many heads are fabricated on this single wafer. The wafer is built using modified standard thin-film processes for fabricating direct access storage device (DASD) heads and modified substrate lapping procedures. The advantages and features of this invention apply equally well to the xe2x80x9cpiggy-backxe2x80x9d MR head structure, the interleaved MR head structure, and to many other thin-film magnetic recording head structures and this invention is to be limited only by the language of the claims set forth below.
It is a purpose of this invention to reduce the cost of separately fabricating, finishing and assembling each recording head by aligning many recording heads at once on both sides of a single wafer and eliminating the requirement for separate finishing of multiple modules for a single recording head. It is a feature of this invention that monolithic MR tape heads are fabricated on both surfaces of a single wafer.
It is another purpose of this invention to eliminate the usual requirement for a conductive shield between track-pair elements of a MR magnetic tape head. It is an advantage of the tape head of this invention that such cumbersome shield is unnecessary because the conductive substrate material provides all necessary shielding. However, it is also an advantage of the method of this invention that a thin film conductive layer such as, for instance, gold or copper, may be formed under the elements on either or both sides of the wafer for improved shielding.
The substrate wafer is processed using standard DASD-like processes and fabricated using standard flat or contoured lapping machines modified to include means for adjusting the angle of inclination between the wafer substrate and the lapping plate to achieve equal stripe height on either side of the wafer substrate. Electronic lapping sensors are provided for both sides of the wafer substrate.
In one aspect, the invention is a process for fabricating thin-film magnetic heads having an air bearing surface (ABS), including the steps of polishing the surface of a first side of a monolithic substrate wafer, forming thereon a first array of magnetic read head structures and magnetic write head structures each having a head gap, polishing the surface of the other side of the monolithic substrate wafer, forming thereon a second array of magnetic read head structures and magnetic write head structures disposed such that a plurality of the magnetic read head gaps on one of the monolithic substrate surfaces are each aligned to form a read/write track-pair with a corresponding one of the magnetic write head gaps on the other monolithic substrate surface, cutting the monolithic substrate to expose the head gaps of a plurality of read/write track-pairs, lapping the ABS to refine the depth of the exposed head gaps, and cutting the monolithic substrate to separate therefrom the thin-film magnetic head.
In a preferred embodiment, the invention is a thin-film magnetic tape head having an air bearing surface (ABS), including a monolithic substrate layer having two sides, a plurality of thin-film read head structures and thin-film write head structures disposed on a first side of the monolithic substrate layer, each having a head gap disposed at the ABS, a plurality of thin-film read head structures and thin-film write head structures disposed on a second side of the monolithic substrate layer, each having a head gap disposed at the ABS and aligned such that a plurality of the read head gaps on one of the monolithic substrate surfaces are each aligned to form a read/write track-pair with a corresponding one of the write head gaps on the other monolithic substrate surface.
In another embodiment, the invention is a magnetic tape drive including a magnetic recording medium having a recording surface, a motor for moving the magnetic recording medium, a thin-film magnetic tape head having an air bearing surface (ABS) and including a monolithic substrate layer having two sides, a plurality of thin-film read head structures and thin-film write head structures disposed on a first side of the monolithic substrate layer, each having a head gap disposed at the ABS, a plurality of thin-film read head structures and thin-film write head structures disposed on a second side of the monolithic substrate layer, each having a head gap disposed at the ABS and aligned such that a plurality of the read head gaps on one of the monolithic substrate surfaces are each aligned to form a read/write track-pair with a corresponding one of the write head gaps on the other monolithic substrate surface, and a head-mount assembly for supporting the magnetic tape head with respect to the magnetic recording medium.
The foregoing, together with other objects, features and advantages of this invention, can be better appreciated with reference to the following specification, claims and the accompanying drawing.